1. Field of the Invention
An object of the present invention is an adapter terminal unit for digital trunks to a standard different to that normally applicable to the digital trunks associated with a time-division switching center which for the purposes of such adaptation is provided with at least one such unit.
2. Description of the Prior Art
As is known, there exist two types of primary PCM multiplex equipment as defined by CCITT Recommendations G.732 and G.733 and it is therefore necessary to be able to connect digital trunks linking equipments conforming to one of these two standards with terminal units of time-division switching centers provided for equipment conforming to the other standard.
It will be remembered that Recommendation G.732 concerns multiplex links with 32 channel time slots operating at 2 048 kbit/s and that Recommendation G.733 concerns links with 24 channel time slots operating at 1 544 kbit/s, both having the same frame duration and the same sampling frequency.
The frame of links conforming to Recommendation G.732 comprise 30 channel time slots for 8-bit samples, the speech signal samples on them being encoded according to the A law, two of the 32 channel time slots of each frame being reserved, channel time slot zero providing for the transmission of the frame alignment signal and channel time slot 16 for the transmission of signaling bits, except in frame zero which is reserved for the multiframe alignment signal.
The multiframe consisting of 16 consecutive frame permits the transmission of four signaling bits for each of the 30 speech channels, eight bits at a time in channel time slot 16 of the last 15 frames.
The frame of links conforming to Recommendation G.733 comprise 24 channel time slots for 8-bit samples and a supplementary bit; the speech signal samples are encoded according to the .mu.-law, those of frames 6 and 12 of the 12-frame multiframe being coded on seven bits only, the eighth bit being used to carry the signaling associated with the channel which comprises it and being conventionally called bit A for the eighth bits of frame 6 and bit B for those of frame 12. The supplementary bit is used alternately for frame synchronization every two frames and for multiframe synchronization or alarm purposes betweentimes.
Common channel signaling may be used with both types of link outlined above.
The adapter terminal unit in accordance with the invention is designed to permit the connection to a time-division switching center of at least one digital trunk normally not connectable to the switching center by virtue of differences of standards between them and this is the case in particular when the switching center is of the type described in French Patent Application No. 2 503 513, designed originally to be connected to digital trunks conforming to Recommendation G.732.
In a switching center of this kind, as is known, the terminal units are conventionally connected to a switching network by multiplex links and they are controlled by a pool of control units also connected to the switching network by multiplex links to permit the exchange of messages between the microprocessors of the terminal units and those of the control units.
The connection to the switching center of digital trunks which are not to the standard provided for therefore implies modification for the purpose of adapting the signals transmitted or to be transmitted over the non-standard digital trunks from or through the switching center.
The objective of the present invention is thus an adapter terminal unit which is identical to the other terminal units of the switching center as seen from the control units of the switching center, which processes the signals on the digital trunk which it connects according to the standards applicable to this digital trunk, as seen from the end of this trunk to which it is not connected, and which handles the various adaptation operations on an autonomous basis, under the control of the microprocessor which it conventionally comprises and with the assistance of automatic devices which are specific to it.
In practice, this adapter terminal unit uses the same control and connection infrastructure as described in French Patent Application No. 82 20407 relating to a mixer terminal unit obviously equipped with different specific equipment.